This invention relates to a method and apparatus for measuring the magnitude of a clamping load applied to a laminated iron core used with an electric machine by calculating the velocity of a sound wave propagating through the laminated iron core in the direction of lamination.
The magnitude of clamping pressure initially applied to the tightened surface of a laminated iron core used with an electric machine gradually decreases due to time-worn conditions such as the driness of insulation varnish baked to both sides of the respective member plates of the core. In extreme cases, the constituent member plates of a laminated iron core are loosened, resulting in the vibration of the core plates, or what is worse, in the occurrence of accidents. As used herein, the term "clamping pressure on the tightened surface" is defined to mean a quotient arrived at by dividing the magnitude of clamping load applied to a laminated iron core by the area of the tightened end face of said core.
Hitherto, the magnitude of clamping load applied to a laminated iron core, therefore, the clamping pressure has been determined by measuring a strain sustained by clamping bolts. The customary pressure measuring process consists in fitting a strain gauge to each of the clamping bolts to measure strains sustained by said bolts themselves due to their action to clamp a laminated iron core; converting measured strains into stresses imparted to said bolts; multiplying the stresses by the total cross sectional areas of the clamping bolts to calculate clamping load imparted to said bolts; and finally determining the magnitude of clamping pressure applied to the tightened end face of a laminated iron core from the product of said multiplication.
However, the conventional pressure measuring process using a strain gauge has the drawbacks that it can not find the magnitude of clamping load applied to the local points on the end face of a laminated iron core, though it may determine the magnitude of average clamping load applied to the entire end face of the core; and the strain gauge is subject to the gradually decreased precision of measurement due to its time-worn deterioration, failing to trace time change in the magnitude of clamping load or clamping load applied to a laminated iron core used with an electric machine over a long period with a constant degree of precision.